1. Field of the Invention
This invention relates to a niobium powder used for a capacitor having excellent leakage current characteristics; a sintered body produced from the niobium powder; and a capacitor having the sintered body.
2. Description of the Related Art
Capacitors to be incorporated in electronic apparatuses such as portable phones and personal computers are demanded to have a small size and a high capacitance. Among such capacitors, a tantalum capacitor has been widely used, in view of high capacitance relative to its size, and excellent performance. Generally, in the tantalum capacitor, a sintered body of tantalum powder is used as a positive electrode, and therefore, in order to increase the capacitance of the capacitor, the weight of the sintered body must be increased.
When the weight of the sintered body is increased, the capacitor necessarily becomes larger in size and fails to satisfy the demand for a small-sized capacitor. In order to solve this problem, a capacitor containing a sintered body of a powdery material having a dielectric constant higher than that of tantalum has been studied. Niobium and titanium are mentioned as examples of the powdery material having a high dielectric constant.
However, a sintered body of the above-described material has a high specific leakage current index. Since niobium and titanium have high dielectric constants, a capacitor having high capacitance can be produced from these materials, but lower specific leakage current index is required in order to produce a capacitor of high reliability. Specific leakage current index, i.e., leakage current per unit capacitance, can be used to evaluate whether high capacitance can be obtained while maintaining leakage current at a practically permissible low level.
The specific leakage current index is determined as follows. A sintered body having a dielectric layer formed thereon by electrolytic oxidation is prepared, and 70% of formation voltage is continuously applied to the sintered body for three minutes. The leakage current during the application of voltage is divided by the product of formation voltage during electrolytic oxidation and capacitance of the sintered body. Thus, the specific leakage current index is expressed by the following formula:Specific leakage current index=[LC/(C×V)]Wherein LC: Leakage current, C: Capacitance, and V: Formation Voltage.
In the case of a sintered body of a tantalum powder, a specific leakage current index is not more than 1,500 [pA/(μF·V)], as calculated from capacitance and leakage current described in a catalogue entitled “CAPACITOR GRADE TANTALUM” by Showa Cabot Supermetals K.K. In order to guarantee this value, it is generally accepted that the actual measured value of specific leakage current index must be at most ⅓ to ¼ of the value calculated from the catalogue, and a preferred leakage current index is not more than 400 [pA/(μF·V)]. However, a conventional sintered body of niobium or titanium powder has a specific leakage current index much higher than the preferred leakage current index, and thus a capacitor containing the sintered body of niobium or titanium has poor reliability and is impractical for use.